Continuous method for obtaining butenes from a C4 fraction

ABSTRACT

A continuous process for isolating butenes from a C 4  fraction comprising butanes, butenes and other C 3 -C 5 -hydrocarbons by extractive distillation using a selective solvent (LM), comprising a first process stage I in a scrubbing zone (E) and a second process stage II in a degassing zone (A), wherein the liquid or a substream of the liquid is taken off from the degassing zone (A) at a theoretical plate located one or more theoretical plates below the feed point for the bottom stream (LM/C 4 H 8 ) from the scrubbing zone (E), heated and/or vaporized by indirect heat exchange with the hot bottom stream (LM) from the degassing zone (A) and returned to the degassing zone (A) at the same theoretical plate or above this, with the theoretical plate from which the liquid or substream of liquid is taken off being selected so that the total energy requirement in the process stages I and II is minimized, is proposed.

The present invention relates to a continuous process for isolatingbutenes from a C₄ fraction by extractive distillation using a selectivesolvent.

The term C₄ fraction refers to mixtures of hydrocarbons havingpredominantly 4 carbon atoms per molecule. C₄ fractions are obtained,for example, in the production of ethylene and/or propylene by thermalcracking of a petroleum fraction such as liquefied petroleum gas,naphthyl or gas oil. C₄ fractions are also obtained in the catalyticdehydrogenation of n-butane and/or n-butene. C₄ fractions generallycomprise butanes, n-butene, isobutene, 1,3-butadiene, together withsmall amounts of other hydrocarbons, and also butynes, in particular1-butyne(ethylacetylene) and butenyne(vinylacetylene). The 1,3-butadienecontent is generally from 10 to 80% by weight, preferably from 20 to 70%by weight, in particular from 30 to 60% by weight, while the content ofvinylacetylene and ethylacetylene generally does not exceed 5% byweight.

The fractionation of C₄ fractions is a complicated distillation problembecause of the small differences in the relative volatilities of thecomponents. Fractionation is therefore carried out by extractivedistillation, i.e. a distillation with addition of a selective solvent(also referred to as extractant) which has a boiling point higher thanthat of the mixture to be fractionated and increases the differences inthe relative volatilities of the components to be separated.

Many processes are known for the fractionation of C₄ fractions by meansof extractive distillation using selective solvents. In all of them, thegaseous C₄ fraction to be fractionated is brought into countercurrentcontact with the liquid selective solvent under appropriatethermodynamic conditions, generally at low temperatures, frequently atroom temperature or at slightly elevated temperature, and at atmosphericpressure, so that the selective solvent is loaded with the components ofthe C₄ fraction for which it has a relatively high affinity, i.e.unsaturated or multiply unsaturated components, while the saturatedcomponents remain in the vapor phase and are taken off at the top. Theunsaturated or multiply unsaturated components are subsequentlyfractionally liberated from the laden solvent stream, i.e. released asgas from the selective solvent, in one or more further process stepsunder suitable thermodynamic conditions, i.e. at higher temperatureand/or lower pressure, compared to the first process step. The degassedsolvent is, after being cooled, generally in an integrated heat systemin which the heat is utilized for increasing the temperature of the feedstream to be fed to degassing, recycled to the first process step, i.e.to the extractive distillation of the C₄ fraction. Such processes areknown, for example, from DE-A 198 188 10 or DE-A 27 24 365.

It is an object of the present invention to provide a process forisolating butenes from a C₄ fraction by extractive distillation using aselective solvent, which process is particularly efficient andeconomical. In particular, the amounts of energy required and thecapital costs should be low in this process.

The achievement of this object starts out from a continuous process forisolating butenes from a C₄ fraction comprising butanes, butenes andpossibly traces of other hydrocarbons by extractive distillation using aselective solvent, in which the C₄ fraction is, in a first process stageI, separated in a scrubbing zone into which the C₄ fraction is fed ingaseous or liquid form and the selective solvent is fed in liquid formabove the feed point of the C₄ fraction into a butane-containing topstream and a bottom stream comprising the selective solvent laden withthe butenes and possibly traces of other hydrocarbons, and the bottomstream is, in a second process stage II, separated in a degassing zoneto which energy is fed via a bottom vaporizer and which is at a highertemperature and/or lower pressure than the scrubbing zone into a topstream comprising the butenes and any traces of other hydrocarbons and abottom stream comprising the selective solvent, with the heat of thebottom stream from the degassing zone being utilized for increasing thetemperature in the degassing zone.

In the process of the present invention, the liquid or a substream ofthe liquid is taken off from the degassing zone at a theoretical platelocated one or more theoretical plates below the feed point for thebottom stream from the scrubbing zone, heated and/or vaporized byindirect heat exchange with the hot bottom stream from the degassingzone and returned to the degassing zone at the same theoretical plate orabove this, with the theoretical plate from which the liquid orsubstream of liquid is taken off being selected so that the total energyrequirement in the process stages I and II is minimized.

The present process can in principle be applied to any C₄ fraction, butit is particularly advantageous to use C₄ fractions which have arelatively high proportion of butenes as starting mixture.

For the purposes of the present invention, traces of other hydrocarbonsare proportions by weight of other hydrocarbons which do not adverselyaffect the specifications of the products obtained from the C₄ fractionin subsequent use.

Starting materials which can advantageously be used are, for example, C₄fractions from an oil refinery, from FCC (Fluidized Catalytic Cracking)plants, which generally have a composition of from 20 to 70% by weightof butanes, from 30 to 80% by weight of butenes together with otherC₃-C₅-hydrocarbons as balance, particularly preferably C₄ fractionscomprising 42% by weight of butanes, 56% by weight of butenes and 2% byweight of other C₃-C₅-hydrocarbons.

A typical C₄ fraction from an FCC plant has the following composition,in % by weight:

propane  0.3 propene  1.2 n-butane 12 i-butane 30 1-butene 14 i-butene10 trans-2-butene 15.5 cis-2-butene 16.5 1,3-butadiene  0.5.

Another C₄ fraction which can advantageously be used in the presentprocess is raffinate 1 from a butadiene plant. This is preferably useddirectly without further intermediate treatment.

In butadiene plants, 1,3-butadiene is isolated from C₄ fractions inwhich it is present, with the C₄ fractions used typically havingcompositions in % by weight in the following ranges:

1,3-butadiene from 10 to 80 butenes from 10 to 60 butanes from 5 to 40other C₄-hydrocarbons and from 0.1 to 5 other hydrocarbons, inparticular C₃- and C₅-hydrocarbons from 0 to a maximum 5.

In butadiene plants, the C₄ fraction to be fractionated is firstlybrought in gaseous form into countercurrent contact with the liquidselective solvent in an extraction zone in which the 1,3-butadiene andfurther hydrocarbons for which the selective solvent has a higheraffinity than for 1,3-butadiene are essentially completely absorbed bythe selective solvent but the components for which the selective solventhas a lower affinity, in particular the butanes and the butenes, mostlyremain in the gas phase. This gas phase is taken off as top stream andis frequently referred to as raffinate 1. In the process of DE 198 18810, the raffinate 1 is the top stream designated as Gbc from theextractive distillation column EI in FIGS. 1 and 2.

In the process of DE-A 27 24 365, the raffinate 1 is the top stream fromthe main scrubber.

An illustrative composition for raffinate 1 in % by weight is shownbelow:

n-butane 17 i-butane 6 1-butene 29 i-butene 36 trans-2-butene 6cis-2-butene 6 1,3-butadiene ≦0.01

The present separation task can be carried out using selective solventswhose affinity for hydrocarbons increases with the presence of doublebonds and further with the presence of conjugated double bonds andtriple bonds, preferably dipolar, particularly preferably dipolaraprotic, solvents. To simplify the choice of materials of constructionfor the apparatus, preference is given to substances which arenoncorrosive or have a low corrosivity.

Selective solvents which are suitable for the process of the presentinvention are, for example, butyrolactone, nitriles such asacetonitrile, propionitrile, methoxypropionitrile, ketones such asacetone, furfural, N-alkyl-substituted lower aliphatic acid amides suchas dimethylformamide, diethylformamide, dimethylacetamide,diethylacetamide, N-formylmorpholine, N-alkyl-substituted cyclic acidamides (lactams) such as N-alkylpyrrolidones, in particularN-methylpyrrolidone. In general, use is made of alkyl-substituted loweraliphatic acid amides or N-alkyl-substituted cyclic acid amides.Dimethylformamide, acetonitrile, furfural and especiallyN-methylpyrrolidone are particularly advantageous.

It is also possible to use mixtures of these solvents with one another,for example N-methylpyrrolidone with acetonitrile, mixtures of thesesolvents with cosolvents such as water and/or tert-butyl ethers, forexample methyl tert-butyl ether, ethyl tert-butyl ether, propyltert-butyl ether, n-butyl or isobutyl tert-butyl ether.

A particularly useful selective solvent is N-methylpyrrolidone, in thepresent text referred to as NMP for short, preferably in aqueoussolution, advantageously with from 0 to 20% by weight of water, inparticular from 7 to 10% by weight of water, particularly preferably8.3% by weight of water.

Process Stage I

In process stage I, a C₄ fraction is subjected to extractivedistillation in a scrubbing zone by feeding the C₄ fraction in gaseousor liquid, preferably in gaseous form and the selective solvent inliquid form above the feed point of the C₄ fraction into the scrubbingzone. In this countercurrent contact of C₄ fraction and solvent, the C₄fraction is separated into a top stream comprising the saturatedcomponents, i.e. the components for which the selective solvent has alower affinity, predominantly butanes, and a bottom stream whichcomprises the solvent laden with components for which the selectivesolvent has a higher affinity than for the butanes, predominantlybutenes and any further hydrocarbons. Preferably the C₄ fraction is fedin gaseous form into scrubbing zone, in its lower region.

The scrubbing zone is generally configured as a column. There are inprinciple no restrictions regarding the separation-active internalswhich can be used in this: it is equally possible to use trays, randompacking or structured packing. The column advantageously has from 10 to80, preferably from 20 to 30, theoretical plates, in particular 26theoretical plates.

Above the feed point for the selective solvent in the upper region ofthe column, there is preferably a backscrubbing zone comprising from 3to 5 trays, in which residual selective solvent is scrubbed out by meansof the runback condensed at the top of the column.

The column pressure in the scrubbing zone is dependent on thetemperature of the cooling medium in the condenser at the top of thecolumn (well water, river water, seawater, refrigerant such as liquidpropylene, liquid ammonia or brine). It is generally from 1 to 15 bar,frequently from 2 to 10 bar, preferably 5.4 bar. The temperature in thecolumn is, on the basis of the abovementioned pressure values, set so asto give suitable thermodynamic conditions under which the selectivesolvent becomes laden with the components of the C₄ fraction for whichit has a greater affinity than for the butanes while the butanes in theC₄ fraction remain in the gas phase. The temperature at the top of thecolumn is typically in the range from about 30 to 60° C.

Process Stage II

The bottom stream from the scrubbing zone is, in process stage II,separated in a degassing zone at a higher temperature and if appropriatelower pressure compared to the scrubbing zone into a top streamcomprising the butenes and any traces of other hydrocarbons and a bottomstream comprising the selective solvent. Here, the heat of the bottomstream from the degassing zone is utilized by means of indirect heatexchange to increase the temperature of a liquid stream taken off fromthe degassing zone.

In the degassing zone, the thermodynamic conditions have to be set sothat degassing of the hydrocarbons, in particular the butenes and anyfurther C₃-C₅-hydrocarbons, from the selective solvent occurs. Ingeneral, if NMP containing from about 7 to 10% by weight of water isused as selective solvent, temperatures at the bottom in the range from150 to 160° C. and pressures in the range from atmospheric pressure to10 bar absolute, preferably 1.5 bar absolute, are necessary for this.

As regards the configuration in terms of apparatus, the degassing zonecan, like the scrubbing zone, be a column which can in principle beequipped with any type of separation-active internals. Preference isgiven to using separation-active internals which have a lowsusceptibility to fouling or are easy to clean, in particular trays.

The column preferably has from 1 to 30 theoretical plates, in particularfrom 2 to 8 theoretical plates, particularly preferably four theoreticalplates.

As in the case of the scrubbing zone, the degassing zone is preferablyprovided in the region above the inlet for the feed stream withbackscrubbing trays for selective solvent entrained in the vapor stream,in general from 3 to 5 trays.

At the bottom of the degassing zone A, hot selective solvent is takenoff as bottom stream. This is cooled in an integrated heat system, i.e.by utilizing its heat content within the process, and recycled toprocess stage I, i.e. to the scrubbing zone.

According to the present invention, the heat of the hot bottom streamfrom the degassing zone A is utilized particularly efficiently by meansof a particular way of carrying out the process so that the total energyrequirement for the process is minimized.

For this purpose, the liquid or a substream of the liquid is taken offfrom the degassing zone A at a theoretical plate located one or moretheoretical plates below the feed point for the feed stream to thedegassing zone, heated and/or vaporized by indirect heat exchange withthe hot bottom stream from the degassing zone and returned to thedegassing zone at the same theoretical plate from which the stream hadbeen taken off.

Preferably the heat of the bottom stream from the degassing zone is usedin addition in a washing zone, by taking off the liquid or a substreamof the liquid from a theoretical plate in the washing zone, situated oneor more theoretical plates below the feed point of the stream of theselective solvent, preferably below the feed point of the C₄ fraction,heating and/or vaporizing it with the hot bottom stream from thedegassing zone and returning it to the same theoretical plate or aboveit into the washing zone, with the theoretical plate from which thestream or substream is taken off being selected so that the total energyrequirement in the process stages I and II is minimized.

In a preferred process variant, the liquid stream or substream taken offis subjected to expansion evaporation to give a gaseous phase and aliquid phase and the gaseous and liquid phases are subsequently returnedto the same theoretical plate from which the liquid stream or substreamhad been taken off or the gaseous part of the liquid stream or substreamwhich was taken off is returned to a theoretical plate situated one ormore theoretical plates above the theoretical plate from which theliquid stream or substream had been taken off.

The inventors have recognized that for each degassing zone there is, asa function of the feed composition, the temperature and pressureconditions, the number of theoretical plates and the prescribedspecification for the desired product taken off as top stream, aparticular theoretical plate at which indirect heat exchange with thehot bottom stream from the degassing zone is most advantageous, becausehere the least amount of energy has to be supplied from the outside tothe bottom vaporizer of the degassing zone, i.e. the total energyrequirement in the process stages I and II is minimized. If the liquidis taken off from a theoretical plate lower down, the temperaturedifference between this and the bottom stream is low as a result of thetemperature profile in the degassing zone and little heat can thereforebe transferred. On the other hand, if the liquid is taken off at atheoretical plate higher up, the following considerations apply: thelargest quantity of heat can be transferred between the feed stream tothe degassing zone and the hot bottom stream because the temperaturedifference is greatest. However, this is likewise not the mosteconomical utilization of the energy of the bottom stream, since itresults in introduction of more energy than is necessary for theseparation task at this point. This excess energy has to be removedeither by means of an unnecessarily high reflux ratio at the condenserat the top of the degassing zone or via an additional cooler.

If the hot solvent has not yet been cooled sufficiently by indirect heatexchange with the liquid taken off from the degassing zone to be able tobe recycled to the extraction zone, the heat content which is stillavailable can be utilized at another point in the process, preferably inthe bottom vaporizer of the extraction zone of process stage I.

In a preferred process variant, the scrubbing zone and degassing zoneare located in a single column. As a result, the capital costs andoperating costs are significantly lower and the plant is safer tooperate.

The invention is illustrated below with the aid of a drawing andexamples.

In the figures:

FIG. 1 schematically shows a preferred plant for carrying out theprocess of the present invention,

FIG. 2 schematically shows a further preferred plant in which thescrubbing zone and the degassing zone are both located in a singlecolumn.

In the figures, identical reference numerals refer to identical orcorresponding features.

FIG. 1 shows a scrubbing zone E for extractive distillation which isconfigured as a column, with an inlet for the liquid solvent LM in theupper region of the column and an inlet for the gaseous C₄ fraction,stream C₄, in the lower region of the column. A top stream, C₄H₁₀,comprising predominantly the butanes is take off from the top of thecolumn and a bottom stream, stream LM/C₄H₈, comprising solvent ladenwith, in particular, butenes and traces of other hydrocarbons is takenoff at the bottom. The top stream is condensed in a condenser W2 at thetop of the column. Part of this is preferably returned as runback to thetop of the column. A heat exchanger W1 is located at the bottom of thecolumn. To set the temperature of the selective solvent LMappropriately, a heat exchanger W3, which is preferably operated bymeans of water, can be provided. The bottom stream LM/C₄H₈ from thescrubbing zone E is introduced into a degassing zone A in its upperregion. Energy is supplied from the outside to the degassing zone A viathe bottom vaporizer W5. The top stream from the degassing zone A iscondensed in the condenser W6 at the top of the column, and part of itis returned as runback to the top of the column and the remainder istaken off as desired product, stream C₄H₈, comprising predominantlybutenes. The hot bottom stream LM from the degassing zone A, whichcomprises predominantly the solvent, transfers part of its heat contentby indirect heat exchange in the heat exchanger W4 to the liquid whichis taken off from the degassing zone A at a theoretical plate locatedbelow the inlet for the feed stream and is returned to the degassingzone A after heating.

FIG. 2 schematically shows a plant in which the scrubbing zone E and thedegassing zone A are both located in a single column.

Liquid solvent LM is fed into the upper region of the upper section of acolumn, which is configured as a scrubbing zone E, and gaseous C₄fraction, stream C₄, is fed into the lower region of this section. A topstream, C₄H₁₀, comprising predominantly the butanes is taken off fromthe column, condensed in a condenser W2 at the top of the column andpart of the condensate is returned as runback to the top of the column.The liquid from the lower region of the scrubbing zone E flows down intothe lower section of the column, which represents the degassing zone A.

Energy is supplied from the outside to the degassing zone A via thebottom vaporizer W5. A stream is taken off from the upper region of thedegassing zone, condensed in a condenser W6 and part of the condensateis returned as runback to the degassing zone A and the remainder istaken off as desired product, stream C₄H₈, comprising predominantlybutenes. The hot bottom stream LM from the degassing zone A, whichcomprises predominantly the solvent, transfers part of its heat contentby indirect heat exchange in the heat exchanger W4 to the liquid whichis taken off from the degassing zone A and, after heating, returned tothe degassing zone A.

Customary trays, ordered packing, random packing or the like can be usedas separation-active internals.

The invention is illustrated further below with the aid of an example:

In a plant as shown schematically in FIG. 1, having 30 theoreticalplates in a scrubbing zone E, a gaseous C₄ fraction, stream C₄, havingthe composition shown below was fed at a flow rate of 13 666 kg/h to the9^(th) theoretical plate counted from the bottom upward in the column,and a liquid solvent stream LM having the composition shown below, ineach case in % by weight, was fed to the 27^(th) theoretical plate.

Composition of the stream C₄:

n-butane 17.1 i-butane 6.4 n-butene 27.8 i-butene 33.8 trans-2-butene8.6 cis-2-butene 6.23 1,3-butadiene 0.07

Composition of the stream LM:

NMP 91.7 Water 8.3

Three theoretical backscrubbing plates were located in the column abovethe feed point for the solvent stream LM. The temperature of the streamC₄ was 41.7° C., the temperature of the stream LM was 34° C. and thepressure at the top of the column was 4.05 bar.

The top stream from the scrubbing zone E was condensed in a heatexchanger W2 and part of the condensate was returned as runback to thetop of the column and the remainder was taken off as stream C₄H₁₀. Thestream C₄H₁₀ comprised 95% of butanes, i.e. 25.6% by weight of n-butane,69.4% by weight of i-butane, balance impurities, predominantly n-butene,trans-2-butene and water.

The bottom stream from the scrubbing zone E, LM/C₄H₈, comprised solventladen with, in particular, butenes and traces of other hydrocarbons andhad, by way of example, the following composition in % by weight:

n-butane 0.15 i-butane 0.30 n-butene 1.7 i-butene 2.1 trans-2-butene0.53 cis-2-butene 0.39 water 7.8 NMP 87.3

The stream LM/C₄H₈ was introduced as feed stream at a temperature of55.4° C. into a degassing zone A at the fourth, i.e. uppermost,theoretical plate. The top stream from the degassing zone A wascondensed in a condenser W6 and the condensate was partly returned asrunback to the degassing zone A and the remainder was taken off asstream C₄H₈ comprising predominantly butenes and having the compositionshown below.

Composition of the stream C₄H₈ in % by weight:

n-butene 32.5 i-butene 40.0 trans-2-butene 10.1 cis-2-butene 7.3Balance: impurities.

Energy was supplied from the outside to the degassing zone A via thebottom vaporizer W5. The hot bottom stream LM from the degassing zone A,which comprised predominantly the solvent, transferred part of its heatcontent by indirect heat exchange in the heat exchanger W4 to the liquidwhich was in each case taken off from the degassing zone A at differenttheoretical plates and, after heating, returned to the degassing zone A.For example, liquid streams were in each case taken from the first,second, third or fourth theoretical plate of the degassing zone A andheated by heat integration, i.e. by indirect heat exchange with the hotsolvent stream LM taken off from the bottom of the degassing zone A, andreturned to the same theoretical plate.

The energy which had to be supplied from the outside to the bottomvaporizer W5 of the degassing zone A was as follows:

Location of heat integration Energy requirement (theoretical plate) inmegawatt 1 10.7 2 7.0 3 6.2 4 6.4 Feed stream 6.9

The experiments showed that the heat requirement for the plant, i.e. theenergy which has to be supplied from the outside, can be minimized ifthe heat integration is carried out at the appropriate theoreticalplate, in the present case the third theoretical plate which is oneplate below the inlet for the feed stream. Heat integration into thefeed stream is likewise unfavorable.

1. A continuous process for isolating butenes from a C₄ fraction whichcomprises butanes, butenes and optionally traces of other hydrocarbonsby extractive distillation using a selective solvent; the processcomprising: feeding the C₄ fraction in gaseous or liquid form into ascrubbing zone, feeding the selective solvent in liquid form into thescrubbing zone above the feed point of the C₄ fraction, separating theC₄ fraction into a first top stream comprising butane and a first bottomstream comprising the selective solvent laden with the butenes andoptionally traces of other hydrocarbons, separating the first bottomstream into a second top stream comprising the butenes and said optionaltraces of other hydrocarbons and a second bottom stream comprising theselective solvent in a degassing zone, wherein energy is fed into thedegassing zone via a bottom vaporizer, the temperature of the degassingzone is at a higher temperature and/or a lower pressure than thescrubbing zone, the heat of the second bottom stream from the degassingzone is utilized for increasing the temperature in the degassing zone, afirst liquid or a substream of the first liquid is taken off from thedegassing zone at a theoretical plate located one or more theoreticalplates below the feed point for the first bottom stream from thescrubbing zone, heated and/or vaporized by indirect heat exchange withthe second bottom stream from the degassing zone and returned to thedegassing zone at or above the theoretical plate, wherein thetheoretical plate from which the liquid or substream of liquid is takenoff is selected to minimize the total energy requirement in the process.2. The process as claimed in claim 1, wherein the C₄ fraction (C₄) is ingaseous form, and is fed into the scrubbing zone in the lower partthereof.
 3. The process as claimed in claim 1, wherein a second liquidor a substream of the second liquid is taken off from the scrubbing zonefrom a theoretical plate located one or more theoretical plates belowthe feed point for the stream of selective solvent and below the feedpoint for the C₄ fraction, heated and/or vaporized by indirect heatexchange with the second bottom stream from the degassing zone andreturned to the scrubbing zone at or above the theoretical plate,wherein the theoretical plate from which the liquid or substream ofliquid is taken is selected to minimize the total energy requirement inthe process.
 4. The process as claimed in claim 1, wherein the selectivesolvent is selected from the group consisting of N-methylpyrrolidone,dimethylformamide, acetonitrile, furfural, mixture ofN-methylpyrrolidone and at least one cosolvent, mixture ofdimethylformamide and at least one cosolvent, mixture of acetonitrileand at least one cosolvent, mixture of furfural and at least onecosolvent, and mixtures thereof.
 5. The process as claimed in claim 4,wherein the selective solvent comprises N-methylpyrrolidone and theN-methylpyrrolidone comprises from 0 to 20% by weight of water.
 6. Theprocess as claimed in claim 1, wherein the first liquid or the substreamof the first liquid from the degassing zone is returned to the sametheoretical plate from which the liquid or the substream of the liquidwas taken off.
 7. The process as claimed in claim 1, wherein the liquidstream or substream taken off is subjected to expansion evaporation togive a gaseous phase and a liquid phase and the gaseous and liquidphases are subsequently returned to the same theoretical plate fromwhich the liquid stream or substream was taken off or the gaseous partof the liquid stream or substream which was taken off is returned to atheoretical plate located one or more theoretical plates above thetheoretical plate from which the liquid stream or substream was takenoff.
 8. The process as claimed in claim 1, wherein the number oftheoretical plates in the scrubbing zone is from 10 to 80, and thenumber of theoretical plates in the degassing zone is from 1 to
 30. 9.The process as claimed in claim 1, wherein the scrubbing zone and thedegassing zone are both located in a single column.
 10. The process asclaimed in claim 5, wherein N-methylpyrrolidone comprises from 7 to 10%by weight of water.
 11. The process as claimed in claim 5, whereinN-methylpyrrolidone comprises 8.3% by weight of water.
 12. The processas claimed in claim 3, wherein the first liquid or the substream of thefirst liquid from the degassing zone and/or the second liquid or thesubstream of the second liquid from the scrubbing zone is returned tothe same theoretical plate from which the liquid or the substream of theliquid was taken off.
 13. The process as claimed in claim 8, wherein thenumber of theoretical plates in the scrubbing zone is from 20 to 30 andthe number of theoretical plates in the degassing zone is from 2 to 8.14. The process as claimed in claim 8, wherein the number of theoreticalplates in the scrubbing zone is 26 and the number of theoretical platesin the degassing zone is 4.